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The massive amounts of heat and pollution that rise from the world’s cities both delay and stimulate the fall of precipitation, cheating some areas of much-needed rain and snow while dousing others, scientists said.

The findings support growing evidence that urbanization has a sharp and alarming effect on the climate, and those changes can wreak havoc with precipitation patterns that supply life’s most precious resource: water.

“These are going to become big issues,” said Steve Burian of the University of Utah.

Details were presented Thursday and Friday at the fall meeting of the American Geophysical Union.

In California, eastward-blowing pollution induces a precipitation deficit across the Sierra Nevada mountain range equal to about 1 trillion gallons of water a year, said Daniel Rosenfeld of Hebrew University in Jerusalem.

The Sierra Nevada is a major source of water for much of California, which relies on it to supply its cities and farms.

“It amounts to significantly less amounts of water,” said Rosenfeld, who has noted similar pollution-linked deficits in Israel.

Opposite effect in Atlanta, Houston
The warmth and grit generated in urban areas can have the opposite effect on local precipitation and actually boost rainfall levels in large cities like Atlanta and Houston.

During the past 60 years, while Houston has grown to become the nation’s fourth-largest city, scientists have measured increased amounts of rain in areas downwind of the urban core during hot, humid summer months, Burian said.

“The majority of evidence is pointing to some sort of urban modification,” he said, adding that more research is needed.

Cities produce large amounts of a class of pollutants called aerosols, which include tiny particles of dust and the byproducts of the combustion of diesel and other fossil fuels.

Atmospheric levels of the pollutant are closely tied to levels of human activity. In New York City, measurements made between June and September 2001 showed that aerosol levels regularly grew during the work week, with a noticeable spike on Wednesdays, then decreased on the weekend, said Menglin Jin of the University of Maryland at College Park. She attributes the midweek spike to a sharp increase in diesel truck traffic.

When hoisted skyward, the microscopic pollutants act as multiple surfaces on which the moisture in clouds can condense to form tiny droplets. That can prevent or delay the formation of larger raindrops that more readily fall from the sky as rain.

In Southern California, a 24 percent decrease in the amount of rainfall measured since 1890 in the town of Cuyamaca appears linked to aerosol pollution wafting from San Diego, roughly 40 miles to the southwest, Rosenfeld said.

Cities also generate and trap tremendous amounts of heat and are on average one to 10 degrees warmer than surrounding undeveloped areas. That heat also changes the dynamics of clouds.

In more humid cities, urbanization appears to invigorate summer storm activity by allowing clouds to build higher and larger before unleashing torrential rains, Burian said. That appears to be the case in Houston.